1. Technical Field
This disclosure relates to noise reduction in electrical oscillators.
2. Description of Related Art
Over the past few decades, integrated electronic systems have witnessed an exponential increase in performance and reduction in cost. The majority of electronic systems rely on an accurate frequency source (sometime referred to as a reference clock). The advancements of wireless and wired communications would not be possible without accurate frequency synthesizers that are at the core of synchronous and coherent transceivers. The performance of these systems is directly related to the quality and accuracy of the frequency synthesis. For instance, the bit-error-rate (BER) of a communication scheme reduces as the spectral purity (or phase noise) of the frequency synthesizer that is used for coherent detection of the incoming signal is improved. Reference clocks are also central to all synchronous processors (e.g., virtually all of today's personal computers) and networking schemes.
In the fields of electronics, radio Frequency (RF) systems, and optics there has always been a quest for achieving the purest signal (i.e., lowest-jitter clock, lowest phase noise RF source, and smallest line-width laser, respectively) in a small form-factor and without using excessive power. Phase-noise, timing-jitter, and spectral line-width all represent the same physical phenomena. Ideally, an oscillator generates a pure periodic waveform. The most common signal is a pure sinusoid. In frequency domain, a pure sinusoid is represented by a pair of delta functions, having value only at the oscillation frequency (and its negative value) and zero elsewhere. In real-world implementations though, all practical oscillators have been shown to be noisy. While amplitude noise is usually not a concern, the phase noise can be.
In time domain, oscillator noise is typically referred to as timing jitter. Jitter represents the deviation in oscillator period from the nominal value due to noise. For instance, in a 1 GHz oscillator with 10 ps cycle-to-cycle rms jitter, the oscillation period at every cycle changes by 1% on the average. Timing jitter is more suitable for clock characterization in synchronous communication, processing, and networking schemes. For instance, as the computing or communication speed increase, the jitter should be lower for subsystems to maintain synchronization.
In frequency domain, oscillator noise is typically referred to as phase noise (or spectral line-width). The frequency spectrum of a noisy oscillator will not be a single delta function at the oscillation frequency; rather it will spread to neighboring frequencies as well. Phase noise measures this noise-induced frequency spreading. Phase noise is the relative signal power at an offset frequency (due to spreading) to the oscillator power. For instance, in a 1 GHz oscillator with −120 dBc/Hz at 1 MHz offset frequency, the power of spread signal at 1 GHz±1 MHz in 1 Hz bandwidth is −120 dB lower compared with the total oscillator power.
Physics and mathematics show that these requirements are countervailing. Most oscillators and lasers include a resonator and an active (gain) block. For low phase noise, the Quality Factor (Q) of the resonator must be high. Quality Factor is a measure of stored energy in the resonator relative to its loss. Historically, the majority of oscillator (and laser) research has focused on achieving the highest possible resonator Q.
In the context of electrical oscillators, unfortunately, extremely high-Q (Q>100) resonators at GHz range are not compatible with standard silicon processing technologies. This is due to the loss of typical materials that are used in a standard silicon process and also the small available volume to store energy in an Integrated Circuit (IC). Therefore, in order to realize low-phase-noise RF oscillators and low-jitter high-speed clocks, a common practice is to use a Phase-Locked Loop (PLL) where the frequency and phase of a GHz integrated oscillator are locked (synchronized) with those of a low-frequency low-phase-noise reference. The low frequency reference is often a crystal-based (e.g., quartz) or a Surface Acoustic Wave (SAW)-based oscillator which is among the very few components that is not integrated with the rest of electronics on the same silicon chip. There is a big advantage in terms of cost and footprint to generate low-phase-noise signals without relying on an off-chip crystal reference.
The predominant method to generate a reference clock is to rely on mechanical resonators such as Quartz crystal or Surface Acoustic Wave (SAW)-based resonator clocks. The frequency of these clocks is typically in the MHz range (below GHz).
Frequency multipliers can be used to generate higher frequencies at GHz. The downside of such a scheme is that the generated frequency cannot be tuned. Although fixed-frequency references may be sufficient in some wire-line synchronous communication, processing, and networking schemes, tunable sources are needed in RF transceivers.
Ultra low-phase-noise microwave oscillators are composed of a very high-Q microwave resonator connected in positive feedback to a low-noise gain element. Low-loss optical fibers have been used to implement high-Q delay-line resonators in an electro-optical feedback configuration to realize low phase-noise microwave oscillators. However, in such systems, the quality factor of the actives used in gain element dominates the overall quality factor. Hence, improvement in phase noise beyond certain limit is not possible.
In a more common scheme, Phase locked loops (PLL) are used to lock the frequency and phase of a poor GHz oscillator (integrated on the chip) with those of a high-quality low-frequency clock (such as a Quartz). PLL also allows frequency programmability in the context of Frequency Synthesizers. Typically, the frequency tuning resolution of a PLL-based frequency synthesizer is limited (e.g., a few KHz). Also, as common with all feedback systems the frequency switching time in PLL-based systems is slow.
Direct Digital Synthesizers (DDS) store the exact sinusoid waveform in a memory and read it using an accurate clock. Depending on the speed at which different memory cells are read. Different frequencies can be generated with a very high accuracy (e.g., 0.01 Hz) and with fast speed (rapid frequency hopping when needed). The power consumption of DDS solutions are very high (several Watts in the GHz range), which makes them unusable for most consumer products (e.g., portable devices). They are commonly used in laboratory instrumentation and in some military systems.